WEDNESDAY, April 1 (HealthDay News) -- Smoking by mothers has replaced infants sleeping on their stomachs as the greatest modifiable risk factor for sudden infant death syndrome, or SIDS, Australian researchers suggest.

They found that when mothers smoke, the sleep arousal process of infants, which awakens them in response to a life-threatening situation, is altered, increasing the risk for SIDS.

The study included 12 healthy, full-term infants born to mothers who smoked an average of 15 cigarettes a day. The infants' arousal responses during daytime sleep were compared with those of 13 healthy infants of nonsmoking mothers. All the children were assessed three times: at 2 to 4 weeks of age, 2 to 3 months, and 5 to 6 months.

The results showed that infants who had been exposed to smoke had reduced sub-cortical activation to cortical arousal. They also had lower rates of full cortical arousals from sleep and higher rates of sub-cortical activations than infants of nonsmoking mothers.

The study also identified a dose-dependent relationship between cortical activation rates and levels of infant urinary cotinine, a nicotine metabolite. Infants with the highest levels of smoke exposure had the lowest levels of cortical arousal.

Decreased cortical arousals from sleep have been observed in infants who later died of SIDS, noted senior investigator Rosemary Horne, scientific director of the Ritchie Centre for Baby Health Research at Monash University in Melbourne.

"Our study suggests that maternal smoking can impair the arousal pathways of seemingly normal infants, which may explain their increased risk for SIDS," Horne said in a news release from the American Academy of Sleep Medicine.